The goal of this study is to increase scientific knowledge of the demographic and behavioral characteristics of female prostitutes, especially as they bear on the heterosexual transmission of HIV infection and other sexually transmitted diseases. The study will draw upon interview data collected between May 1990 and February 1991 from a probability sample of 998 street prostitutes in Los Angeles County, a subsample of 638 of whom provided serological data that were tested for markers for HIV-1 infection and past or present infection with syphilis and hepatitus B. Preliminary analyses indicate that the data are of good quality and the sample presents a little studied population that is demographically and behaviorally at high risk for transmission of STDs. Almost 69 percent of respondents were black, and 9 percent were Hispanic. Respondents had worked an average of 6.4 years on the street and saw a mean of 30.2 clients a week; 30 percent had also worked in a crack house. Approximately 75 percent sail they would agree not to use a condom some of the time in response to a specific request for a client; 20 percent had used intravenous drugs in the six months preceding the interview. Sampling histories and interview data were used to construct weights that support inferences about the size of Los Angeles County's work force of female street prostitutes. The proposed research will extend these analyses (10 to analyze the characteristics of women involved in street prostitution in Los Angeles County during the field period; (2) develop weighted incidence estimates for encounters between street prostitutes and their clients and profile these encounters; (3) analyze the most recent prostitute/client transaction with respect to client, prostitute, and situational variables associated with engaging in risky sexual acts.